Mechanism for starvation avoidance while maintaining cache consistency in computer systems

ABSTRACT

A computer system comprises a system memory, a first active device configured to access data stored in the system memory, and a second active device configured to access data stored in said system memory. The first active device is configured to generate a read-to-share transaction corresponding to a load operation. In response to receiving an invalidating transaction subsequent to receiving the read-to-share transaction, the first active device selectively allows the load operation to complete depending upon an indication of whether the load operation is a critical load. In one embodiment, an active device comprises a processing unit and an interface controller coupled to receive a load request corresponding to a load operation initiated by said processing unit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to the field of multiprocessor computer systems and, more particularly, to starvation avoidance in computer systems.

[0003] 2. Description of the Related Art

[0004] Multiprocessing computer systems include two or more processors that may be employed to perform computing tasks. A particular computing task may be performed upon one processor while other processors perform unrelated computing tasks. Alternatively, components of a particular computing task may be distributed among multiple processors to decrease the time required to perform the computing task as a whole.

[0005] A popular architecture in commercial multiprocessing computer systems is a shared memory architecture in which multiple processors share a common memory. In shared memory multiprocessing systems, a cache hierarchy is typically implemented between the processors and the shared memory. In order to maintain the shared memory model, in which a particular address stores exactly one data value at any given time, shared memory multiprocessing systems employ cache coherency. Generally speaking, an operation is coherent if the effects of the operation upon data stored at a particular memory address are reflected in each copy of the data within the cache hierarchy. For example, when data stored at a particular memory address is updated, the update may be supplied to the caches that are storing copies of the previous data. Alternatively, the copies of the previous data may be invalidated in the caches such that a subsequent access to the particular memory address causes the updated copy to be transferred from main memory.

[0006] Shared memory multiprocessing systems generally employ either a snooping cache coherency protocol or a directory-based cache coherency protocol. In a system employing a snooping protocol, coherence requests are broadcast to all processors (or cache subsystems) and memory through a totally ordered address network. Each processor “snoops” the requests from other processors and responds accordingly by updating its cache tags and/or providing the data to another processor. For example, when a subsystem having a shared copy of data observes a coherence request for exclusive access to the block, its copy is typically invalidated. Likewise, when a subsystem that currently owns a block of data observes a coherence request to that block, the owning subsystem typically responds by providing the data to the requestor and invalidating its copy, if necessary. By delivering coherence requests in a total order, correct coherence protocol behavior is maintained since all processors and memories observe requests in the same order.

[0007] In a standard snooping protocol, requests arrive at all devices in the same order, and the access rights of the processors are modified in the order in which requests are received. Data transfers occur between caches and memories using a data network, which may be a point-to-point switched network separate from the address network, a broadcast network separate from the address network, or a logical broadcast network which shares the same hardware with the address network. Typically, changes in ownership of a given cache block occur concurrently with changes in access rights to the block.

[0008] A potential problem associated with shared memory multiprocessing systems is referred to as starvation. For example, in a computer system employing a standard snooping protocol, a processor P1 which is attempting to perform a load to a cache block which it does not have must issue a request for shared access to the cache line. This processor P1 will gain the right to read from this block when it receives its own request, and it can lose that right as soon as it receives a later request from a different processor P2 for exclusive access to the same block. In particular, P1 may lose its access rights due to a request from P2 which it receives before it receives the data. As a result, P1 would not be able to perform the load and would have to issue another request for shared access to the block. The access rights granted by this request could also be lost (e.g., due to a request from yet another processor P3) before it received the data. In this manner, the processor could fail to perform the load an unbounded number of times and thus could starve.

[0009] The starvation problem is typically solved in systems that utilize standard snooping protocols by placing incoming address packets in a queue. The packets are removed from the queue in the order they are placed in, and they affect the processor's access rights only when they are moved from the queue. When a processor P1's own request is at the front of its queue, but it has not yet received the data for this request, it waits until the data is received before removing its request from the queue. As a result, starvation on loads is avoided because the data must be received before a later, invalidating process from another processor is received.

[0010] Unfortunately, the standard snooping protocol suffers from a significant performance drawback. In particular, the requirement that access rights of processors change in the order in which snoops are received may limit performance. For example, a processor may have issued requests for cache blocks A and B, in that order, and it may receive the data for cache block B (or already have it) before receiving the data for cache block A. In this case the processor must typically wait until it receives the data for cache block A before using the data for cache block B, thus increasing latency. The impact associated with this requirement is particularly high in processors that support out-of-order execution, prefetching, multiple core per-processor, and/or multi-threading, since such processors are likely to be able to use data in the order it is received, even if it differs from the order in which it was requested.

[0011] In systems that implement a directory-based protocol rather than a snooping protocol, both the address network and the data network are typically point-to-point, switched networks. When a processor requests a cache block, the request is sent to a directory which maintains information regarding the processors that have copies of the cache block and their access rights. The directory then forwards the request to those processors which must change their access rights and/or provide data for the request (or if needed, the directory will access the copy of the cache block in memory and provide the data to the requestor). Since there is no way of knowing when the request arrives at each processor to which it is sent, all processors that receive the request must typically acknowledge reception by providing data or sending an acknowledge (ACK) message to either the requestor or the directory, depending on the protocol.

[0012] Typical systems that implement a directory-based protocol may be associated with various drawbacks. For example, systems that employ directory-based protocols may suffer from starvation in a manner similar to that discussed previously. In addition, such systems may suffer from high latency due to the requirement that requests go first to a directory and then to the relevant processors, and/or from the need to wait for acknowledgment messages. Still further, when a large number of processors must receive the request (such as when a cache block transitions from a widely shared state to an exclusive state), all of the processors must typically send ACKs to the same destination, thus causing congestion in the network near the destination of the ACKs and requiring complex logic to handle reception of the ACKs. Finally, the directory itself may add cost and complexity to the system.

[0013] It may accordingly be desirable to provide an efficient cache consistency protocol that does not require that access rights or processors change in the order in which requests are received while avoiding starvation.

SUMMARY OF THE INVENTION

[0014] Various computer systems and methods that maintain cache consistency while avoiding starvation are disclosed. In one embodiment, a computer system comprises a system memory, a first active device configured to access data stored in the system memory, and a second active device configured to access data stored in the system memory. The first active device is configured to generate a read-to-share transaction corresponding to a load operation. In response to receiving an invalidating transaction subsequent to receiving the read-to-share transaction, the first active device selectively allows the load operation to complete depending upon an indication of whether the load operation is a critical load.

[0015] In one embodiment, an active device comprises circuitry to generate a read-to-share transaction corresponding to a load operation, wherein, in response to receiving a foreign invalidating transaction subsequent to receiving the read-to-share transaction, the circuitry selectively allows the load operation to complete depending upon an indication of whether the load operation is a critical load.

[0016] In another embodiment, an active device comprises a processing unit and an interface controller coupled to receive a load request corresponding to a load operation initiated by said processing unit. The interface controller is configured to generate a read-to-share transaction in response to the load request. In addition, in response to receiving a foreign invalidating transaction subsequent to receiving the read-to-share transaction, the interface controller selectively allows the load operation to complete depending upon an indication of whether the read-to-share transaction corresponds to a critical load.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:

[0018]FIG. 1 is a block diagram of a multiprocessing computer system.

[0019]FIG. 2 is a chart illustrating various transactions which may be supported in one embodiment of a computer system.

[0020]FIG. 3 illustrates data packet transfers in accordance with one embodiment of a computer system.

[0021]FIG. 4 illustrates various non-cacheable transaction types that may be supported in one embodiment of a computer system.

[0022]FIGS. 5A and 5B illustrates types of access rights and ownership status that are possible in one embodiment of a computer system.

[0023]FIG. 5C illustrates possible combinations of access rights and ownership status.

[0024]FIG. 6 is a chart illustrating ownership transitions in response to particular transactions in one embodiment of a computer system.

[0025] FIGS. 7A-7F illustrate exemplary coherence operations which may be implemented in one embodiment of a computer system.

[0026]FIG. 8 is a block diagram illustrating details of one embodiment of each of the processing subsystems of FIG. 1.

[0027]FIG. 9 is a block diagram illustrating further details regarding one embodiment of each of the processing subsystems of FIG. 1.

[0028]FIG. 10 is a diagram illustrating multiple coherence transactions initiated for the same cache block.

[0029]FIG. 11 is a diagram illustrating communications between active devices in accordance with one embodiment of a computer system.

[0030]FIG. 12 is a block diagram of another embodiment of a multiprocessing computer system.

[0031] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Turning now to FIG. 1, a block diagram of one embodiment of a computer system 140 is shown. Computer system 140 includes processing subsystems 142A and 142B, memory subsystems 144A and 144B, and an I/O subsystem 146 interconnected through an address network 150 and a data network 152. In the embodiment of FIG. 1, each of processing subsystems 142, memory subsystems 144, and I/O subsystem 146 are referred to as a client device. It is noted that, although five client devices are shown in FIG. 1, embodiments of computer system 140 employing any number of client devices are contemplated. Elements referred to herein with a particular reference number followed by a letter will be collectively referred to by the reference number alone. For example, processing subsystems 142A-142B will be collectively referred to as processing subsystems 142.

[0033] Generally speaking, each of processing subsystems 142 and I/O subsystem 146 may access memory subsystems 144. Devices configured to perform accesses to memory subsystems 144 are referred to herein as “active” devices. Each client in FIG. 1 may be configured to convey address transactions on address network 150 and data on data network 152 using split-transaction packets. Processing subsystems 142 may include one or more instruction and data caches which may be configured in any of a variety of specific cache arrangements. For example, set-associative or direct-mapped configurations may be employed by the caches within processing subsystems 142. Because each of processing subsystems 142 within node 140 may access data in memory subsystems 144, potentially caching the data, coherency must be maintained between processing subsystems 142 and memory subsystems 144, as will be discussed further below.

[0034] Memory subsystems 144 are configured to store data and instruction code for use by processing subsystems 142 and I/O subsystem 146. Memory subsystems 144 preferably comprise dynamic random access memory (DRAM), although other types of memory may be used. Each address in the address space of computer system 140 may be assigned to a particular memory subsystem 144, referred to as the home subsystem of the address.

[0035] I/O subsystem 146 is illustrative of peripheral devices such as, for example, an input-output bridge, a graphics device, a networking device, etc. In various embodiments, I/O subsystem 146 may include a cache memory subsystem similar to those of processing subsystems 142 for caching data associated with addresses mapped within one of memory subsystems 144.

[0036] In one embodiment, data network 152 may be a logical point-to-point network. Data network 152 may be implemented as an electrical bus, a circuit-switched network, or a packet-switched network. In embodiments where data network 152 is a packet-switched network, packets may be sent through the data network using techniques such as wormhole, store and forward, or virtual cut-through. In a circuit-switched network, a particular client device may communicate directly with a second client device via a dedicated point-to-point link that may be established through a switched interconnect mechanism. To communicate with a third client device, the particular client device utilizes a different link as established by the switched interconnect than the one used to communicate with the second client device. Data network 152 may implement a source-destination ordering property such that if a client device C1 sends a data message D1 before sending a data message D2 and a client device C2 receives both D1 and D2, C2 will receive D1 before C2 receives D2.

[0037] In the embodiment of FIG. 1, address network 150 implements a broadcast network in which address transactions are conveyed to all client devices. Address network 150 may be embodied physically using a point-to-point, switched network configured to broadcast address transactions. Address network 150, as well as data network 152, may be implemented using a multi-stage switching hierarchy. In alternative embodiments, address network 150 may be implemented using a common bus structure. Address network 150 may be embodied in hardware that is separate from data network 152, or in hardware that is shared with data network 152.

[0038] Address network 150 accommodates communication between processing subsystems 142, memory subsystems 144, and I/O subsystem 146. Messages upon address network 150 are generally referred to as address packets. When the address packet references a storage location within a memory subsystem 144, the location referenced may be specified via an address conveyed within the address packet upon address network 150. Subsequently, data corresponding to the address packet on the address network 150 may be conveyed upon data network 152. Typical address packets correspond to requests for an access right (e.g., a readable or writable copy of a cacheable coherency unit) or requests to perform a read or write to a non-cacheable memory location. Address packets may be sent by a device in order to initiate a coherence transaction. In the computer system 140 shown in FIG. 1, a coherence transaction may include one or more packets upon address network 150 and data network 152. Typical coherence transactions involve at least one address packet and one or more data packets. These packets implement data transfers, ownership transfers, and/or changes in access privileges. It is noted that address network 150 may serve as an ordering point for transactions involving a given cacheable coherency unit, and thus it is possible to refer to such transactions as occurring “before” or “after” each other. The ordering point for transactions involving non-cacheable coherency units may be the target device of the transaction.

[0039] In one embodiment, read-to-share (RTS) transactions may be initiated by active devices upon address network 150 to request read-only copies of cache blocks. Similarly, read-to-own (RTO) transactions may be initiated by active devices to request writable copies of cache blocks. Other coherence transactions may similarly be initiated by active devices upon address network 150, as desired.

[0040]FIG. 2 is a chart illustrating various transactions, including a description for each, that may be supported by one embodiment of computer system 140. As illustrated, in addition to read-to-share and read-to-own transactions, further coherence transactions that may be supported include read-stream (RS) transactions, write-stream (WS) transactions, write-back (WB) transactions, and write-back shared (WBS) transactions. A read-stream transaction requests a read-once copy of a cache block. A write-stream transaction is a request to write an entire cache block and send the cache block to memory. A write-back transaction is a request to send a cache block from an owning device to memory, where the owning device does not keep a copy. Finally, a write-back shared transaction is a request to send a cache block from an owning device to memory, where the owning device keeps a read-only copy. Active devices may also be configured to initiate other transaction types on address network 150 such as, for example, I/O read and write transactions and interrupt transactions. In one embodiment, a read-to-writeback (RTWB) transaction may also be supported to allow I/O bridges (or other devices) to perform a write to part of a cache block without gaining ownership of the cache block and responding to foreign requests for the cache block.

[0041] It is noted that transactions may be conveyed upon address network 150 in the form of encoded packets including a specified address. Data packets conveyed on data network 152 may be associated with corresponding address transactions using transaction IDs, as discussed below.

[0042] In one embodiment, cacheable transactions result in at least one packet being received by the initiating client on the data network 152. In addition, some transactions may require that a packet be sent from the initiating client on the data network 152. FIG. 3 illustrates data packet transfers on data network 152 that may result from various transactions in accordance with one embodiment of computer system 140. A PRN data packet type is a pull request, sent from the destination of a write transaction to the source of the write transaction, to send data. An ACK data packet type is a positive acknowledgment from an owning device allowing a write stream transaction to be completed. A NACK data packet type is a negative acknowledgment to memory aborting a WB, WBS, or to the initiator aborting an INT transaction.

[0043] When an initiator initiates a transaction, the address packet for that transaction may include a transaction ID formed by the initiator's device ID and a packet ID, which is assigned by the initiator. The DATA, ACK and/or PRN packets that the initiator receives may be routed to the initiator through data network 152 by placing the initiator's device ID in the packets' routing prefix. In addition, the DATA, ACK and/or PRN packets may contain a destination packet ID field which matches the packet ID assigned by the initiator, thus allowing the initiator to match the DATA, ACK, and/or PRN packet to the correct transaction. Furthermore, PRN packets may include a pull ID consisting of the source's device ID and a packet ID, which is assigned by the source (that is, the client which sent the PRN packet). After receiving a PRN packet, the initiator may send a DATA or NACK packet to the source of the PRN. This DATA or NACK packet may be routed by placing the device ID of the source of the PRN in the packet's routing prefix, and may contain a destination packet ID field which allows it to be matched with the correct PRN (in addition, the packet may include a flag which indicates that it was sent in response to a PRN, thus preventing confusion between transaction IDs and pull IDs).

[0044] In one embodiment, the ACK packet which may be sent in response to a WS does not contain any data. The ACK packet may be used to indicate the invalidation of the previous owner. The PRN packet which an initiator receives in response to a cacheable transaction is sent by the memory device that maps the cache block. The DATA or NACK packet which the initiator sends is sent to the memory device that maps the cache block (which is also the source of the PRN received by the initiator).

[0045] As illustrated in FIG. 3, the initiator may receive separate DATA and PRN packets for a RTWB transaction. However, when the owner of the cache block is the memory device that maps the cache block, these two packets would be sent by the same client. Thus, in one embodiment, instead of sending two packets in this situation, a single DATAP packet may be sent which combines the information of a DATA packet and a PRN packet. Similarly, a single PRACK packet, which combines the information of a PRN packet and an ACK packet, may be sent in response to a WS transaction when the owner of the cache block is the memory device that maps the cache block. Finally, in those cases where the initiator is the owner of the cache block, the initiator may not send a DATA or ACK packet to itself (this can be viewed logically as a transmission of a DATA or ACK packet from the initiator to itself which does not leave the initiator). Similarly, in those cases where the initiator is the memory device that maps the cache block, the initiator may not send a PRN packet to itself, nor need it send a DATA or NACK packet to itself.

[0046] In the embodiment of FIG. 1, non-cacheable transactions may similarly result in at least one packet being received by the initiating client from the data network, and some transactions may require that a packet be sent from the initiating client device on the data network. FIG. 4 illustrates various non-cacheable transaction types that may be supported in one embodiment of computer system 140, along with resulting data packet types that may be conveyed on data network 152. The columns in FIG. 4 are indicative of the sequence of packets sent on the address and data networks, in order from left to right.

[0047] The DATA, PRN, or NACK packets that an initiator may receive in response to non-cacheable transaction types are routed to the initiator through data network 152 and may be matched to the correct transaction at the receiver through the use of transaction IDs, as was described for cacheable data transfers. Similarly, the DATA packets that the initiator sends may be routed to their destination and matched to the correct transaction at their destination through the use of pull IDs, as was described for cacheable transactions.

[0048] For RIO and WIO transactions, the DATA, and/or PRN packets which the initiator receives are sent from the client that maps the block. For INT transactions, the PRN or NACK packet which the initiator receives is sent from the target of the interrupt (which may be specified in an address field of the INT packet). When the initiator sends a DATA packet, it sends the DATA packet to the source of the PRN which it received. It is noted that when the initiator would be both the source and destination of a DATA, PRN, or NACK packet, no DATA, PRN, or NACK packet needs to be sent. It is also noted that when an initiator receives a PRN packet in response to an INT transaction, the initiator sends a data packet. When the initiator receives a NACK packet in response to an INT transaction, the initiator may not send any packet on the data network.

[0049] As will be described in further detail below, in one embodiment computer system 140 is configured to satisfy the following two properties (where a packet is defined to be “received” when any changes in ownership status and/or access rights in response to the packet at the receiving processor have been made, if necessary, pursuant to the coherence protocol):

[0050] 1) Ordered broadcasts: if an address broadcast B1 is received before another address broadcast B2 at a processor (or other device) P1, then B1 is also received before B2 at every other processor (and every other active device) P2, and

[0051] 2) Synchronized broadcasts: if a data packet D1 is sent from a processor (or other device) P1 after P1 received an address broadcast B1, and if D1 is received at a processor (or other device) P2, then P2 receives B1 before P2 receives D1.

[0052] Computer system 140 employs a cache coherence protocol to provide a coherent view of memory for clients with caches. For this purpose, state information for each cache block may be maintained in each active device. The state information specifies the access rights of the active device and the ownership responsibilities of the active device.

[0053] The access right specified by the state information for a particular cache block is used to determine whether the client device can commit a given operation (i.e., a load or a store operation) and constraints on where that operation can appear within one or more partial or total orders. In one embodiment, the memory access operations appear in a single total order called the “global order”. In such an embodiment, these constraints upon where an operation can be placed in the global order can be used to support various well-known memory models, such as, for example, a sequentially consistent memory model or total-store-order (TSO), among others.

[0054] The ownership responsibility specified by the state information for a particular cache block indicates whether the client device is responsible for providing data of the cache block to another client that requests it. A client device owns a cache block if it is responsible for providing data to another client which requests it.

[0055] In one embodiment, the coherence protocol employed by computer system 140 is associated with the following properties:

[0056] 1) Changes in ownership status occur in response to the reception of address packets. Sending address packets, sending data packets, and receiving data packets do not affect the ownership status.

[0057] 2) An active device may own a cache block without having the data associated with that ownership responsibility.

[0058] 3) Access rights transition with receiving address packets, sending data packets, and receiving data packets. Sending address packets does not affect the access rights (although it may affect the way in which other packets are processed).

[0059] 4) An active device which has an access right to a cache block always has the data associated with that access right.

[0060] 5) Reception of address packets is not blocked based on the reception of particular data packets. For example, it is possible to receive a local read request packet before the data being requested is also received.

[0061] Since access rights and ownership status can transition separately in the protocol employed by computer system 140, various combinations of coherence states are possible. FIGS. 5A and 5B illustrate types of access rights and ownership status that are possible in one embodiment of computer system 140. FIG. 5C illustrates possible combinations of access rights and ownership status. It is noted that these combinations differ from those of traditional coherence protocols such as the well-known MOSI protocol. It is also noted that other specific forms of access rights may be defined in other embodiments.

[0062] As illustrated in FIG. 5A, the W (Write) access right allows both reads and writes. The A (All-Write) access right allows only writes, and requires that the entire cache block be written. The R (Read) access right allows only reads. The T (Transient-Read) access right allows only reads; however, unlike reads performed under the W or R access rights, reads performed under the T access right may be reordered, as discussed below. Finally, the I (Invalid) access right allows neither reads nor writes. When the system is first initialized, all active devices have the I access right for all cache blocks. As will be discussed further below, when a cache block is in the A access right state, because the entire cache block must be modified, the data contained in the cache block prior to this modification is not needed and may not be present. Instead, an ACK packet, which acts as a token representing the data, must have been received if the data is not present.

[0063] As illustrated in FIG. 5B, an active device may have an 0 (owner) ownership status or an N (non-owner) ownership status with respect to a given cache block. In either state, data corresponding to the cache block may or may not be present in the cache.

[0064] Once an active device has acquired a given access right, it may exercise that access right repeatedly by performing multiple reads and/or writes, until it loses the access right. It is noted that for access rights other than A (All-Write), an active device may neither read nor write the given cache block, even though its access right allows it to do so. The A access right requires that the entire block be written, so the active device must perform at least one write to each byte in the block.

[0065] In the embodiment of FIG. 1, changes in access rights may occur in response to receiving address packets, sending data packets, or receiving data packets. Generally speaking, and as will be described in further detail below, when a request transfers exclusive access to a cache block from a processor P1 to a processor P2, the sending of the data from P1 terminates P1's access right to the block and the reception of the data at P2 initiates its access right. When a request changes exclusive access to a cache block at a processor P1 to a shared state (i.e., each having a read access right) with a processor P2, the sending of the data from P1 terminates P1's write access right (though it can continue to read the cache block) and the arrival of the data at P2 initiates its shared access right. When a request transfers a cache block from a shared state to exclusive access at a processor P2, the access rights at all processors other than P2 and the processor which owns the cache block (if any) are terminated upon reception of the broadcast request, the access right of the processor that owns the cache block (if there is one) is terminated when it sends the data, and the write access right at P2 is initiated once P2 has received the data from the previous owner (or from memory). Finally, when a request adds a processor P2 to a set of processors that is already sharing a cache block, no processor loses access rights and P2 gains the read access right when it receives the data.

[0066] Ownership responsibilities may transition in response to the reception of address packets. In the embodiment of FIG. 1, sending and receiving data packets do not affect ownership responsibilities. FIG. 6 is a chart illustrating ownership transitions in response to particular transactions in one embodiment of computer system 140. In FIG. 6, “previous owner” indicates that ownership is unchanged, “initiator” indicates that the client which initiated the transaction becomes the owner, and “memory” indicates that the memory subsystem 144 which maps the cache block becomes the owner. The owner of the cache block is either an active device or the memory device which maps the cache block. Given any cachable transaction T which requests a data or ACK packet, the client that was the owner of the cache block immediately preceding T will send the requested data or ACK packet. When the system is first initialized, memory is the owner for each cache block.

[0067] The transitioning of access rights and ownership responsibilities of client devices in one embodiment of computer system 140 may be better understood with reference to the exemplary coherence operations depicted in FIGS. 7A-7F. FIG. 7A illustrates a situation in which an active device D1 has a W access right and ownership, and an active device D2 initiates an RTS in order to obtain the R access right. In this case, D1 will receive the RTS packet from D2 through address network 150. In response to the RTS, D1 sends a corresponding data packet (containing the requested data) to device D2. It is noted that D1 can receive additional address and/or data packets before sending the corresponding data packet to D2. When D1 sends the corresponding data packet to D2, D1 loses its W access right, and changes its access right to an R access right. When D2 receives the corresponding data packet, it acquires an R access right (D2's access right was previously I). D1 continues to maintain ownership of the cache block.

[0068]FIG. 7B illustrates a situation in which an active device D1 has a W access right and ownership, and an active device D2 initiates an RTO transaction in order to obtain a W access right. In this case D1 will receive the RTO packet from D2 over address network 150. As a result, D1 changes its ownership status to N (not owner) and sends a corresponding data packet to D2. It is noted, however, that D1 can receive additional address and/or data packets before sending the corresponding data packet to D2. It is also noted that D2 also receives its own RTO via address network 150. When D1 sends the corresponding data packet to D2, D1 loses its W access right, and changes its right to an I access right. When D2 receives its own RTO via address network 150, its ownership status changes to O (owned). When D2 receives the corresponding data packet, it acquires a W access right.

[0069]FIG. 7C illustrates a situation in which an active device D1 has an R access right and ownership, active device D2 also has an R access right (but not ownership), and active device D3 issues an RTO in order to obtain a W access right (D3 may initially have either an I or R access right). In this case, D1 will receive the RTO from D3 via address network 150. As a result, D1 changes its ownership status to N and sends a corresponding data packet to D3. It is noted, however, that D1 can receive additional address and/or data packets before sending the corresponding data packet to D3. When D1 sends the corresponding data packet to D3, D1 changes its access right to an I access right. In addition, D2 will also receive the RTO via address network 150. When D2 receives the RTO, it changes its R access right to an I access right. Furthermore, when D3 receives its own RTO via address network 150, its ownership status is changed to O. When D3 receives the corresponding data packet, it acquires a W access right. It is noted that the corresponding data packet and the local RTO may be received by D3 before the invalidating RTO packet arrives at D2. In this case, D2 could continue to read the cache block even after D3 has started to write to it.

[0070]FIG. 7D illustrates a situation in which an active device D1 has an R access right and ownership, active device D2 has an R access right, and active device D3 issues an RTS in order to obtain the R access right. In this case, D1 will receive the RTS from D3 via the address network 150. As a result, D1 sends a corresponding data packet to D3. When D3 receives the corresponding data packet, its access right changes from an I access right to an R access right. The reception of the RTS at D1 and D2 does not cause a change in the access rights at D1 or D2. Furthermore, no change in ownership results.

[0071] In the case of a WS (Write Stream) transaction, in which an entire cache block is written by an active device and sent to memory, the device performing the WS may receive an ACK packet from the processing subsystem 142 (or memory subsystem 144) that most recently (in address broadcast order) owned the cache block. It is noted that this ACK packet may be in place of a regular data message (and in fact a data message could be used), and that only one such ACK message may be sent in response to the WS.

[0072]FIG. 7E illustrates a situation in which an active device D1 has an R or W access right and ownership, and an active device D2 issues a WS transaction. As shown, the WS transaction is received by D1 as well as the memory subsystem 144 that maps the cache block through address network 150. As a result, D1 sends a corresponding ACK packet to D2. It is noted, however, that D1 can receive additional address and/or data packets before sending the corresponding ACK packet to D2. When D1 sends the corresponding ACK packet to D2, D1 changes its access right to an I access right. When D2 receives the ACK packet from D1, its access right changes to A (All-Write). In addition, the memory subsystem (M) that maps the cache block forwards a PRN packet on data network 152 to D2. When D2 writes to the entire cache block, D2 forwards a data packet to the memory subsystem M. Upon receipt of the WS transaction through address network 150, D1 changes its ownership status to N (not-owned), and the memory subsystem M changes its ownership status to owned.

[0073]FIG. 7F illustrates a situation in which an active device D1 has a W access right and ownership, and issues a WB transaction in order to write back data to memory. The memory subsystem (M) that maps the cache block receives the WB packet through address network 150, and responsively forwards a PRN packet through data network 152 to D1. As a result, D1 sends a corresponding data packet to memory M. It is noted that D1 can receive additional address and/or data packets before sending the corresponding data packet to memory M. When D1 receives its own WB through address network 150, its ownership status changes to N. When D1 sends the corresponding data packet to memory M, its access right is changed to an I access right. Following this sequence of transactions, memory M becomes the owner of the cache block. WBS (write-back shared) transactions may be handled similarly.

[0074] It is contemplated that numerous variations of computer systems may be designed that employ the principle rules for changing access rights in active devices as described above. Such computer systems may advantageously maintain cache consistency while attaining efficient operation. It is noted that embodiments of computer system 140 are possible that implement subsets of the transactions described above in conjunction with FIGS. 7A-7F. Furthermore, other specific transaction types may be supported, as desired, depending upon the implementation.

[0075] It is also noted that variations with respect to the specific packet transfers described above for a given transaction type may also be possible. Additionally, while ownership transitions are performed in response to receipt of address packets in the embodiments described above, ownership transitions may be performed differently as a result of certain coherence transactions in other embodiments.

[0076] In addition, in accordance with the description above, an owning device may not send a corresponding data packet immediately in response to a transaction (such as an RTO or RTS) initiated by another device. In one embodiment, a maximum time period (e.g., maximum number of clock cycles, etc.) may be used to limit the overall length of time an active device may expend before sending a responsive data packet.

[0077] The synchronized broadcast property identified above could be achieved using various mechanisms. For example, the synchronized broadcast property could be achieved by creating a globally synchronous system running on a single clock, and tuning the paths in address network 150 to guarantee that all broadcasts corresponding to a given transaction arrive at all devices upon the same cycle. In such a system, address packets may be received without buffering them in queues. While such implementations are possible, it may instead be desirable to allow for higher communication speeds using source-synchronous signaling, in which a source's clock is sent along with a particular packet. In such implementations, the cycle at which the packet will be received may not be known in advance. In addition, it may further be desirable to provide queues for incoming address packets to allow devices to temporarily receive packets without flow controlling the address network 150.

[0078] Details regarding one implementation of computer system 140 which maintains the synchronized broadcasts property without requiring a globally synchronous system and which allows address packets to be buffered will thus be described next in conjunction with FIG. 8. FIG. 8 is a block diagram illustrating details of one embodiment of each of the processing subsystems 142 of computer system 140. Included in the embodiment of FIG. 8 are a processing unit 702, cache 710, and queues 720A-720D. Queues 720A-720B are coupled to data network 152 via data links 730, and queues 720C-720D are coupled to address network 150 via address links 740. Each of queues 720 comprises a plurality of entries each configured to store an address or data packet. Processing unit 702 is shown coupled to cache 710. Cache 710 may be implemented using a hierarchical cache structure.

[0079] Processing unit 702 is configured to execute instructions and perform operations on data stored in memory subsystems 144. Cache 710 may be configured to store copies of instructions and/or data retrieved from memory subsystems 144. In addition to storing copies of data and/or instructions, cache 710 also includes state information 712 indicating the coherency state of a particular cache block within cache 710, as discussed above. In accordance with the foregoing, if processing unit 702 attempts to read or write to a particular cache block, and cache state info 712 indicates processing unit 702 does not have adequate access rights to perform the desired operation, an address transaction comprising a coherency request may be inserted in address out queue 720D for conveyance on address network 150. Subsequently, data corresponding to the cache block may be received via data-in queue 720B.

[0080] In addition, processing subsystem 142 may receive coherency demands via address-in queue 720C, such as a read-to-own or read-to-share transaction initiated by another active device (or initiated by itself). If, for example, processing subsystem 142 receives a transaction corresponding to a read-to-own request initiated by a foreign device for a memory block, the corresponding memory block may be returned via data out queue 720A (e.g., if the cache block was owned by the processing subsystem 142), and/or the state information 712 for that block may be changed to invalid, as discussed above. Other coherence transactions as well as non-cacheable transactions may similarly be received through address-in queue 720C. Memory subsystems 144 and I/O subsystem 146 may be implemented using similar queuing mechanisms.

[0081] The synchronized broadcasts property described above may be maintained by implementing address network 150 and data network 152 in accordance with certain network conveyance properties and/or by controlling queues 720 according to certain queue control properties. In some embodiments, address network 150 and data network 152 are implemented such that the maximum skew from when any broadcast (conveyed on address network 150) arrives at any first client device to when the same broadcast arrives at any second client device is less than or equal to the minimum latency for any message sent on the data network 152 from the first to the second client device. Such an implementation results in the following Network Conveyance Property (which is identical to the synchronized broadcasts property, except that it is stated in terms of packet arrivals (i.e., when packets arrive at in-queues 720B and 720C) rather than receptions (i.e., when a packet affects ownership status and/or access rights in the receiving device)): If a data packet D1 is sent from a processor P1 to a processor P2, and if a broadcast B1 arrives at P1 before D1 is sent, then BI arrives at P2 before D1 arrives at P2.

[0082] To verify that the Network Conveyance Property holds true for such a system, consider packets B1 and D1 referred to in the Network Conveyance Property stated above, and define symbols for the times at which various events occur, as follows: Symbol Event W Arrival of B1 at P1 X Sending of D1 from P1 Y Arrival of B1 at P2 Z Arrival of D1 at P2

[0083] The symbol ‘<’ is used to indicate that an event occurs before another event, e.g. A<B would indicate that event A occurs before event B. The symbol ‘<=’ is used to indicate that an event occurs before or simultaneously with another event, e.g. A<=B would indicate that event A occurs before or simultaneously with event B. Also, let S represent the maximum amount of time from when any given broadcast arrives at P1 to the time when the given broadcast arrives at P2 and let L represent the minimum amount of time from when a packet sent on the data network from P1 (destined for P2) until that packet arrives at P2. The symbol ‘+’ is used to indicate the addition of time to an event, e.g. A+S would indicate the addition of S to event A.

[0084] Assume that W<X, i.e. B1 arrives at P1 before P1 sends D1. In order to prove that the Network Conveyance Property holds, it must be shown that Y<Z, i.e. B1 arrives at P2 before D1 arrives at P2. Note that Y<=W+S (from the definition of S), W<X (from the assumption above), and S<=L (from the construction of the address network and the data network). Thus, Y<X+L. Furthermore, X+L<=Z (from the definition of L), so it follows that Y<Z (and that the Network Conveyance Property holds).

[0085] In addition to implementing address network 150 and 152 such that the Network Conveyance Property holds, address-in queue 720C and data-in queue 720B are controlled by a queue control circuit 760 such that packets from the address and data networks are placed in the respective queue upon arrival and are removed (and thus received) in the order they are placed in the queues (i.e., on a first-in, first-out basis). Furthermore, no data packet is removed from the data-in queue 720B for processing until all address packets that arrived earlier than the data packet have been removed from the address-in queue 720C.

[0086] In one embodiment, queue control circuit 760 may be configured to store a pointer along with an address packet when it is stored in an entry at the tail of the address-in queue 720C. The pointer indicates the next available entry in the data-in queue 720B (i.e., the entry that the data-in queue 720C will use to store the next data packet to arrive). In such an embodiment, address packets are received (i.e., they affect the access rights of corresponding blocks in cache 710) after being popped from the head of address-in queue 720C. Queue control circuit 760 may be configured to prevent a particular data packet from being received (i.e., processed by cache 710 in such a way that access rights are affected) until the pointer corresponding to the address packet at the head of the address-in queue 720C points to an entry of data-in queue 720B that is subsequent to the entry including the particular data packet. In this manner, no data packet is removed from the data-in queue 720B for processing until all address packets that arrived earlier than the data packet have been removed from the address-in queue 720C.

[0087] In an alternative embodiment, queue control circuit 760 may be configured to place a token in the address-in queue 720C whenever a packet is placed in the data-in queue 720B. In such an embodiment, queue control 760 may prevent a packet from being removed from the data-in queue 720B until its matching token has been removed from the address-in queue 720C. It is noted that various other specific implementations of queue control circuit 760 to control the processing of packets associated with queues 720 are contemplated.

[0088] By controlling address-in queue 720C and data-in queue 720B in this manner and by implementing address network 150 and data network 152 in accordance with the Network Conveyance Property discussed above, computer system 140 may maintain the synchronized broadcasts property. To evidence this, consider packets B1 and D1 as in the definition of the synchronized broadcasts property and define times as follows: Symbol Event W Arrival of B1 at P1 W′ Processing of B1 at P1 X Sending of D1 from P1 Y Arrival of B1 at P2 Y′ Processing of B1 at P2 Z Arrival of D1 at P2 Z′ Processing of D1 at P2

[0089] Assume that W′<X. In order to prove that the synchronized broadcasts property holds, it must be shown that Y′<Z′. Note that W<=W′ (since a packet must arrive before it can be received and processed), so W<X. Therefore, Y<Z (from the Network Conveyance Property). As a result, Y′<Z′ since data packets in the data-in queue 720B cannot overtake address packets in the address-in queue 720C.

[0090] Turning next to FIG. 9, further details regarding an embodiment of each of the processing subsystems 142 of FIG. 1 are shown. Circuit portions that correspond to those of FIG. 8 are numbered identically.

[0091]FIG. 9 depicts an interface controller 900 coupled to processing unit 702, cache 710, and data and address queues 720. Interface controller 900 is provided to control functionality associated with the interfacing of processing subsystem 142 to other client devices through address network 150 and data network 152. More particularly, interface controller 900 is configured to process various requests initiated by processing unit 702 that require external communications (e.g., packet transmissions) to other client devices, such as load and store requests that require the generation of read-to-share and read-to-own transactions. Interface controller 900 is also configured to process communications corresponding to transactions initiated by other client devices. In one particular implementation, interface controller 900 includes functionality to process transactions in accordance with the foregoing description, including that associated with the processing of the coherence operations as illustrated in FIGS. 7A-7F. For this purpose, functionality depicted as transitory state controller 902 is provided within interface controller 900 for processing outstanding local transactions (that is, transactions initiated by processing subsystem 142 that have not reached a stable completed state). To support this operation, information relating to the processing of coherence operations (including state information) may be passed between interface controller 900 and cache 710. Transitory state controller 902 may include multiple independent state machines (not shown), each of which may be configured to process a single outstanding local transaction until completion.

[0092] The functionality depicted by transitory state controller 902 may be configured to maintain various transitory states associated with outstanding transactions, depending upon the implementation and the types of transactions that may be supported by the system. For example, from the exemplary transaction illustrated in FIG. 7B, device D2 enters a transitory state 10 after receiving its own RTO and prior to receiving a corresponding data packet from device D1. Similarly, device D1 enters transitory state WN in response to receiving the RTO from device D2 that is maintained until the corresponding data packet is sent to device D2. In one embodiment, transitory state controller 902 maintains such transitory states for pending local transactions to thereby control the processing of address and data packets according to the coherence protocol until such local transactions have completed to a stable state.

[0093] Referring back to FIG. 5C, it is noted that states WO, RO, RN, and IN are equivalent to corresponding states defined by the well-known MOSI coherence protocol. These four states, in addition to state WN, are stable states. The other states depicted in FIG. 5C are transient and only exist during the processing of a local transaction by interface controller 900. In addition, in one embodiment, the state WN may not be maintained for cache blocks that do not have a local transaction pending since it may be possible to immediately downgrade from state WN to state RN for such cache blocks. As a result, in one particular implementation, only two bits of state information are maintained for each cache block within state information storage 712 of cache 710. Encodings for the two bits are provided that correspond to states WO, RO, RN, and IN. In such an embodiment, transitory state information corresponding to pending local transactions may be separately maintained by transitory state controller 902.

[0094] Various additional transitory states may also result when a coherence transaction is initiated by an active device while a coherence transaction to the same cache block is pending within another active device. For example, FIG. 10 illustrates a situation in which an active device D1 has a W access right and ownership, and an active device D2 initiates an RTO transaction in order to obtain a W access right. When D1 receives the RTO packet from D2 through address network 150, D1 changes its ownership status to N (non-owner). D2 changes its ownership status to O (owner) when it receives its own RTO through address network 150. It may be possible that another active device D3 subsequently issues another RTO to the same cache block which is received by D2 through address network 150 before a corresponding data packet is received at D2 from D1. In this situation, D2 may change its ownership status to N (non-owner) when the second RTO is received. In addition, when D3 receives its own RTO through address network 150, its ownership status changes to O (owner). When a corresponding data packet is received by D2 from D1, D2's access right changes to a write access right. D2 may exercise this write access right repeatedly, as desired. At some later time, a corresponding data packet may be sent from D2 to D3. When the data is received by D3, it acquires a W access right. Such operations and transitory state transitions may be performed and maintained by the functionality depicted by transitory state controller 902, as needed, based upon the types of transactions that may be supported and the particular sequence of packet transmissions and receptions that may occur, as well as upon the particular coherence methodology that may be chosen for a given implementation.

[0095] As illustrated, the interface controller 900 depicted in FIG. 9 may further include a promise array 904. As described above, in response to a coherence transaction, a processing subsystem that owns a cache block may be required to forward data for the cache block to another device. However, the processing subsystem that owns the cache block may not have the corresponding data when the transaction is received. Promise array 904 is configured to store information identifying data packets that must be conveyed to other devices on data network 152 in response to pending coherence transactions as dictated by the coherence protocol.

[0096] Promise array 904 may be implemented using various storage structures. For example, promise array 904 may be implemented using a fully sized array that is large enough to store information corresponding to all outstanding transactions for which data packets must be conveyed. In one particular implementation, each active device in the system can have at most one outstanding transaction per cache block. In this manner, the maximum number of data packets that may need to be forwarded to other devices may be bound, and the overall size of the promise array may be chosen to allow for such maximum number of data promises. In alternative configurations, address transactions may be flow-controlled in the event promise array 904 becomes full and is unable to store additional information corresponding to additional data promises. Promise array 904 may include a plurality of entries, each configured to store information that identifies a particular data packet that needs to be forwarded, as well as information identifying the destination to which the data packet must be forwarded. In one particular implementation, promise array 904 may be implemented using a linked list.

[0097] Turning next to FIG. 11, it is noted that systems that employ general aspects of the coherence protocols described above could potentially experience a starvation problem. More particularly, as illustrated, an active device D1 may request a read-only copy of a cache block to perform a load operation by conveying a read-to-share (RTS) transaction upon address network 150. However, as stated previously, a corresponding data packet may not be conveyed to D1 from D2 (i.e., the owning device) until some time later. Prior to receiving the corresponding data packet, device D1 has the cache block in an I (Invalid) state. It is possible that prior to receiving the corresponding data packet, that a device D3 may initiate an RTO (or other invalidating transaction) that is received by D1 ahead of the corresponding data packet. This situation may prevent device D1 from gaining the read access right to the cache block since the previously received RTO may nullify the effect of the first request. Although device D1 may issue another RTS to again attempt to satisfy the load, additional read-to-own operations may likewise again be initiated by other active devices that continue to prevent device D1 from gaining the necessary access right. Potentially, requests for shared access to a cache block could be nullified an unbounded number of times by requests for exclusive access to the cache block, thus causing starvation.

[0098] Such a starvation situation can be avoided by defining certain loads as critical loads. Generally speaking, a critical load refers to a load operation initiated by an active device that can be logically reordered in the global order without violating program order. In one embodiment that implements a TSO (Total Store Order) memory model, a load operation is a critical load if it is the oldest uncommitted load operation initiated by processing unit 702. To avoid starvation, in response to an indication that an outstanding RTS corresponds to a critical load and receipt of an intervening foreign RTO transaction to the same block (before a corresponding data packet for the RTS is received) transitory state controller 902 may be configured to provide a T (Transient-Read) access right to the cache block upon receipt of the data packet. The T access right allows the load to be satisfied when the data packet is received. After the load is satisfied, the state of the cache block is downgraded to I (Invalid). This mechanism allows critical loads to be logically reordered in the global order without violating program order. The load can be viewed as having logically occurred at some point right after the owner (device D2) sends the corresponding data packet to D1 (or to D3) but before the device performing the RTO (device D3) receives its corresponding data packet. In this manner, the value provided to satisfy the load in device D1 includes the values of all writes prior to this time and none of the values of writes following this time.

[0099] In one particular implementation, processing unit 702 may provide an indication that a load is the oldest uncommitted load when the load request is conveyed to interface controller 900. In another embodiment, a load may be indicated as being a critical load if it is the oldest uncommitted load at the time the local RTS is conveyed on address network 150. In still a further embodiment, a load may be indicated as being a critical load if it is the oldest uncommitted load at the time the foreign invalidating RTO is received.

[0100] It is noted that, in the scenario described in conjunction with FIG. 11, if the RTS is not indicated as being associated with a critical load, transitory state controller 902 may maintain the cache block in the I (Invalid) state (rather than assigning the T state) in response to receiving the corresponding data.

[0101] It is also noted that in systems that implement other memory models, a load operation may be a critical load (i.e., a load operation that can be logically reordered in the global order) when other conditions exist. For example, in a system that implements sequential consistency, a load operation may be defined as a critical load if there are no older uncommitted load or store operations.

[0102] Numerous variations in the system described above are contemplated. For example, while the address network 150 described above implements a broadcast network, other systems are contemplated in which address transactions may be conveyed point-to-point or to subsets of client devices. Such systems may employ general aspects of the protocols described above for transitioning access rights in active devices.

[0103] In addition, it is noted that embodiments are possible in which memory subsystems 144 are integrated with the functionality of processing subsystems 142, as depicted in FIG. 12. In such embodiments, the conveyance of certain packets on the address and/or data networks as discussed above for particular coherence transactions may not be necessary.

[0104] Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

We claim:
 1. A processing system comprising: a processing unit; and an interface controller coupled to receive a load request corresponding to a load operation initiated by said processing unit, wherein said interface controller is configured to generate a read-to-share transaction in response to said load request, and wherein, in response to receiving a foreign invalidating transaction subsequent to receiving said read-to-share transaction, said interface controller selectively allows said load operation to complete depending upon an indication of whether the read-to-share transaction corresponds to a critical load.
 2. The processing system as recited in claim 1 wherein said interface controller is configured to transition access rights to selected cache blocks in response to receiving data packets of corresponding coherence transactions.
 3. The processing system as recited in claim 2 wherein said processing unit provides said indication to said interface controller of whether the read-to-share transaction corresponds to a critical load.
 4. The processing system as recited in claim 1 wherein said read-to-share transaction is indicated as corresponding to a critical load if said load operation is an oldest uncommitted load operation.
 5. The processing system as recited in claim 4 wherein said read-to-share transaction is indicated as corresponding to said critical load in response to determining that said load operation is the oldest uncommitted load when the load request is provided from said processing unit to said interface controller.
 6. The processing system as recited in claim 4 wherein said read-to-share transaction is indicated as corresponding to said critical load in response to determining that said load operation is the oldest uncommitted load at a time when the read-to-share transaction is conveyed on an address network.
 7. The processing system as recited in claim 4 wherein said read-to-share transaction is indicated as corresponding to said critical load in response to determining that said load operation is the oldest uncommitted load at a time when the foreign invalidating transaction is received by said interface controller.
 8. The processing system as recited in claim 1 wherein said interface controller includes a state controller, wherein said state controller is configured to provide a transient read access right to a cache block corresponding to the read-to-share transaction in response to receiving said foreign invalidating transaction and said indication indicating that the read-to-share transaction corresponds to a critical load.
 9. The processing system as recited in claim 8 wherein the state controller is configured to place the cache block in an invalid state in response to receiving said foreign invalidating transaction and said indication indicating that the read-to-share transaction does not correspond to a critical load.
 10. The processing system as recited in claim 8 wherein the state controller is configured to allow the load request to be satisfied in response to providing the transient read access right to the cache block and to invalidate the cache block after the load request is satisfied.
 11. A computer system comprising: a system memory; a first active device configured to access data stored in said system memory, wherein the first active device includes a first cache configured to store data accessed by said first active device; and a second active device configured to access data stored in said system memory; wherein said first active device is configured to generate a read-to-share transaction corresponding to a load operation, and wherein, in response to receiving an invalidating transaction subsequent to receiving said read-to-share transaction, said first active device selectively allows said load operation to complete depending upon an indication of whether the load operation is a critical load.
 12. The computer system as recited in claim 11 wherein said first active device is configured to transition access rights to selected cache blocks in response to receiving data packets of corresponding coherence transactions.
 13. The computer system as recited in claim 11 wherein said load operation is indicated as being a critical load if said load operation is an oldest uncommitted load operation.
 14. The computer system as recited in claim 13 wherein said load operation is indicated as being said critical load in response to determining that said load operation is the oldest uncommitted load at a time when the read-to-share transaction is conveyed on an address network by the first active device.
 15. The computer system as recited in claim 13 wherein said load operation is indicated as being said critical load in response to determining that said load operation is the oldest uncommitted load at a time when the invalidating transaction is received by said first active device.
 16. The computer system as recited in claim 11 wherein said first active device is configured to provide a transient read access right to a cache block corresponding to the read-to-share transaction in response to receiving said invalidating transaction and said indication indicating that the load operation is a critical load.
 17. The computer system as recited in claim 16 wherein the first active device is configured to place the cache block in an invalid state in response to receiving said invalidating transaction and said indication indicating that the load operation is not a critical load.
 18. The computer system as recited in claim 17 wherein the first active device is configured to allow the load operation to complete in response to providing the transient read access right to the cache block and to invalidate the cache block after the load operation is satisfied.
 19. An active device comprising circuitry to generate a read-to-share transaction corresponding to a load operation, wherein, in response to receiving a foreign invalidating transaction subsequent to receiving said read-to-share transaction, said circuitry selectively allows said load operation to complete depending upon an indication of whether the load operation is a critical load.
 20. The active device as recited in claim 19 wherein said load operation is indicated as being a critical load in response to determining that said load operation is the oldest uncommitted load at a time when the read-to-share transaction is conveyed on an address network.
 21. A method comprising: generating a read-to-share transaction corresponding to a load operation; in response to receiving a foreign invalidating transaction subsequent to receiving said read-to-share transaction, selectively allowing said load operation to complete depending upon an indication of whether the load operation is a critical load.
 22. The method as recited in claim 21 further comprising: a processing unit generating a load request corresponding to the load operation; an interface controller generating the read-to-share transaction in response to said load request; in response to receiving the foreign invalidating transaction subsequent to receiving said read-to-share transaction, said interface controller selectively allowing said load operation to complete depending upon said indication of whether the read-to-share transaction is a critical load.
 23. The method of claims 23 wherein said load operation is indicated as being a critical load if said load operation is an oldest uncommitted load operation. 